In the field of printing technology in general, digitization of printing process has recently been in progress. This technology involves creation of images and documents or manuscripts in digitized form on a personal computer or reading images on a scanner to digitize the image data and directly making a printing plate based on the digital data thus obtained. This allows labor-saving in the whole printing process and facilitates high precision printing.
Hitherto, there has been generally used as a plate for use in printing a so-called PS plate which has anodized aluminum as a hydrophilic non-image part and a hydrophobic image part formed by curing a light-sensitive resin on a surface of the non-image part. To prepare a printing plate using the PS plate, a plurality of steps are necessary so that making of plates takes a long time and incurs high costs. Therefore, currently it is difficult to promote a reduction in time of printing process and a reduction in cost of printing. In particular, this is the major factor of an increase in printing costs in the case of making a small number of prints.
When printing of one picture pattern is completed, the plate has to be exchanged by a new one before next printing can be performed and the used plates have been disposed of. Further, with PS plates, it is impossible to directly make printing plates based on digital data and the making of printing plates is a hindrance to the progress of digitization of printing process in order to achieve labor-saving or high precision printing.
To obviate the above disadvantages with PS plates, several methods have been proposed to facilitate preparation of printing plates in accordance with the digitization of printing process, and some of them have been commercialized. For example, there are known methods which comprise providing a PET film having coated thereon a laser absorbing layer such as a carbon black layer and a silicone resin layer in order and imagewise irradiating the film with laser light to generate heat in the laser absorbing layer to burn off the silicone resin layer by the heat to prepare a printing plate, methods which comprise coating an oleophilic laser absorbing layer on an aluminum plate and a hydrophilic layer on the oleophilic laser absorbing layer and irradiating the hydrophilic layer with laser light to burn it off to make a printing plate, and the like. Although these methods allow preparation of printing plates directly based on digital data, in these methods, when the printing of one picture pattern is over, the printing plate must be exchanged by a new one before the next printing can be performed. Therefore, printing plates once used must be disposed of and in this regard, the above methods are the same as the method which uses the PS plate. That is, the cost of printing increases accordingly. From the viewpoint of protection of global environment which recently has come to be frequently advocated, the disposal of plates which have used once is undesirable.
In recent years, printing plate materials which comprise a photocatalyst and which can be renewed have been disclosed (Japanese Patent Applications, First Publications (Kokai), Nos. Hei 10-250027, Hei 11-245533, and Hei 11-249287, etc.). However, these publications do not explicitly describe the sensitivity of the photocatalysts to ultraviolet rays. In addition, these publications do not explicitly describe the time required for writing an image, or describe that it requires 1 hour to write an image (see Japanese Patent Application, First Publication (Kokai), No. Hei 11-249287), which is far from a practical level. Moreover, with regard to the renewal process, a heat treatment (130 to 200° C.×1 to 5 hours; see Japanese Patent Application, First Publication (Kokai), No. Hei 11-245533) and a process in which an additional photo-reactive layer is formed by lamination are disclosed. However, these renewal processes take too much time, or the renewal time is not explicitly described. Thus, at the present time, no renewal process has yet reached a practical level.